1. Field of the Invention
The present invention relates to a system for detecting a fully closed state of a throttle valve which is available for idle speed control, fuel cut-off control and the like in internal combustion engines, and more particularly to a fully closed-throttle valve detecting system for internal combustion engines which can stably maintain the decision criteria for the full closure of a throttle valve even if the opening degree of the throttle valve varies at the vicinity of its fully closed position.
2. Description of the Related Art
In internal combustion engines, there have heretofore been employed a system of detecting a totally closed condition of a throttle valve taken in response to the release of an accelerator pedal in implementing idle speed feedback control and the like. In this instance, as a rule an idle switch is used as a means to detect the totally closed state of the throttle valve.
However, since there is a need for the idle switch to always come into an on-state when the throttle valve get into the totally closed condition, the idle switch is designed to turn on not only when the throttle valve takes the totally closed condition but also when being in a slightly open condition.
For this reason, in a case where, for example, the vehicle driver lightly places his foot on the accelerator pedal while the engine is in the idle operating condition, the idle switch does not turn into the off-state but maintaining its on-state (totally closed condition). At this time, the throttle valve takes a slightly open condition with the idle switch being not in the off state, and hence the intake air quantity to the engine sometimes increases so that the engine speed increases.
Accordingly, if such a full-closure detecting system is applied to a prior idle speed control, then the control is such that the idle condition (the full closure of the throttle valve) is decided from the turning-on state of the idle switch and the bypass air quantity is feedback-controlled so that the engine speed becomes equal to a target idle speed.
In this case, in the case that the vehicle driver lightly places his foot on the accelerator pedal as mentioned above, even in a state where the engine speed increases while the idle switch is in the turn-on condition, there results in continuous feedback control. Thus, in a case where the driver releases the accelerator pedal to allow the throttle valve to get into the fully closed condition after continuing to place his foot thereon, since the bypass air quantity already decreases at the time of the operation of the accelerator pedal, difficulty is encountered to keep the coincidence of the engine speed with the target idle speed at shifting to the full closure, with the result that there is a possibility that an engine stall arises or the idle speed exceedingly lowers.
FIG. 7 is a timing chart showing one example of a variation of an engine speed Re in relation to a variation of an opening degree (depressing amount of an accelerator pedal) of a throttle valve, and exemplifies the relationship among an idle signal AS (indicative of a state of an idle switch), the opening degree .theta. of the throttle valve, an bypass air quantity Qb, and the engine speed Re. In FIG. 7, Qbo denotes a basic air quantity for the bypass air quantity Qb at idle operation, and Reo depicts a target idle speed for the engine speed Re.
In this instance, there are shown a behavior (see time t2) taken in a case where the vehicle driver lightly places his foot on the accelerator pedal (time t0) when the idle signal AS is in the on-state and the accelerator pedal is released to allow the throttle valve to get into a fully closed state (.theta.=0) (time t1) after the driver continues to press the accelerator pedal as it is (throttle opening degree .theta.=.theta.a), and a behavior (see time t4) arising in a case where the accelerator pedal is released to permit the throttle valve to come into a fully closed condition (.theta.=0) (time t3) after the accelerator pedal is on pressing to cause the idle signal AS to come into the off-state and to stop the idle condition (non-idling state: throttle opening degree .theta.=.theta.b) and then returning to a state where the driver lightly places his foot thereon (lightly footed state) and being kept in that condition (throttle opening degree .theta.=.theta.a).
As obvious from FIG. 7, when the idle signal AS takes its on-state (the throttle valve is substantially in the fully closed condition), the feedback control starts on the basis of the detection of the increase in the engine speed Re even if the throttle valve takes the throttle opening degree .theta.a resulting from the driver lightly placing his foot on the accelerator pedal, thus controlling the bypass air quantity Qb. Thus, if the accelerator pedal shifts from the lightly footed state (throttle opening degree .theta.=.theta.a) to the throttle valve full-closure side (.theta.=0) (see time t1), the engine speed Re rapidly decreases (see time t2).
Further, if the vehicle driver once presses the accelerator pedal to turn off the idle signal AS (throttle opening degree .theta.=.theta.b) before returning it to the lightly footed state (throttle opening degree .theta.=.theta.a), the feedback control for the bypass air quantity Qb begins to suppress the engine speed Re. Accordingly, if the accelerator pedal is subsequently returned to the throttle valve full-closure side (.theta.=0) (see time t3) , the engine speed Re rapidly decreases (see time t4). Even in this case, the troubles such as an engine stall sometimes take place.
For resolution of these disadvantages, there has been proposed a technique as disclosed in Japanese Patent Published No. 6-100129. However, this prior technique can not sufficiently cope with a problem that, when the vehicle driver releases the accelerator pedal, the engine stall arises or the idle speed lowers due to the lack of the bypass air quantity Qb immediately after that release of the accelerator pedal.
More specifically, according to the prior technique, the minimum value (coincident with the throttle opening degree at the full-closure of the throttle valve) of the throttle opening degree .theta. is detected through a throttle sensor so that, even if the idle signal AS is in the on-state, the feedback control stops on the decision that the accelerator pedal is in a lightly footed condition when the throttle opening degree .theta. exceeds the minimum value by a given value. In addition, for compensating for the variation of the throttle valve full-closure detecting system with the passage of time, the minimum value of the throttle opening degree .theta. is initialized in the non-idling condition and the minimum value is again detected at shifting to the idling condition.
Thus, although the prior technique can avoid the rapid decrease (see time t2 in FIG. 7) of the engine speed Re which arises when the accelerator pedal is released from the lightly footed state while the idle signal AS is in the on-state, it can not prevent the rapid decrease (see time t4) of the engine speed Re which arises when the accelerator pedal returns from the non-idling state (a large throttle opening degree .theta.b) to the lightly footed state (a small throttle opening degree .theta.a in the substantially full-closure state) and is kept in that condition and then the driver completely releases the accelerator pedal.
Moreover, the throttle valve full-closure detecting system noted above is also applicable to fuel supply control for cutting off the fuel when the throttle valve is in the fully closed condition (full-closure fuel cut-off). As a rule, the fuel supply control is for the purpose of cutting off the fuel with a view to improving the fuel consumption when the engine speed Re exceeds a predetermined speed for decision (the lower limit of the fuel cut-off area) and the throttle valve is in the fully closed state.
In the case that the throttle valve full-closure detecting system is used in the fuel supply control application, even if the fuel cut-off is made while the accelerator pedal is in the lightly footed state, there is a need for the fuel cut-off decision speed to be set high in order to avoid the occurrence of the hatching of the engine speed Re resulting from the return to the fuel cut-off state. Accordingly, in this instance, the fuel cut-off operating area becomes narrow, thus resulting in considerably counteracting the original full-closure fuel cut-off effects.
Since the prior throttle valve full-closure detecting system for an internal combustion engine is, as described above, designed such that the idle switch detects the fully closed state irrespective of the throttle valve being in a slightly open condition (.theta.=.theta.a), in the case of being applied to the feedback control for the idle speed, the system provides a problem in that, owing to the shortage
the air quantity, the engine stall takes place or the engine speed Re lowers at the time of the idling operation.
In addition, since the technique disclosed in the Japanese Patent Published No. 66-100129, for example, is such that the lightly footed state of the accelerator pedal is decided on the condition that the throttle opening degree .theta. after character becomes greater by a given value than the minimum throttle opening degree so as to inhibit the feedback control for the idle speed, the lowering of the engine speed Re similarly occurs when the driver frees the accelerator pedal from the footing after returning it from the non-idling state (.theta.=.theta.b) exhibiting a large throttle opening degree .theta. to the lightly footed state (.theta.=.theta.a) and then keep it in that condition.
Moreover, in the case that the system is applied to the fuel supply control to cut of fuel in an area where the engine speed is above a predetermined decision speed when the throttle valve is in the fully closed condition, the fuel cut-off operating region becomes narrow, for that the fuel cut-off decision speed is set to a high value in order to avoid the hatching of the engine speed Re, with the result that the original full-closure fuel cut-off effects deteriorate.